1. Field of the Invention
The present invention relates generally to a piston for a reciprocating type internal combustion engine and more specifically to a piston which features a construction which improves the lubrication of the thrust surfaces of the same.
2. Description of the Prior Art
JP-A-60-111047 discloses a piston of the nature shown in FIGS. 14 and 15. In this arrangement the piston 1 has a head section 2 which is formed with piston ring grooves 3a, 3b and 3c; a curved surface skirt section 4 and piston bosses 5, 5. The construction further includes thrust surfaces 7 which are defined on the outer periphery of the piston surface and which are adapted to engage the cylinder bore wall and guide the piston as it ascends to and descends from top dead center (TDC).
The skirt 4 is formed with a first crecent-shaped recess 8 which merges in part with the lower of the three piston ring grooves 3c and which is located immediately above the thrust surface 7, and a second essentially inverted U-shaped recess 9 in the lower portion of the skirt section 4.
The first recess 8 and the strip-like surface 10 of the second recess 9 are respectively formed at predetermined angles as will be appreciated from FIGS. 14 and 15. As the piston reciprocates up and down, lubricating oil tends to be retained in the first and second recesses 8, 9 in a manner which prevents the same from flying off and which enables its use to form a friction reducing oil film between the cylinder bore wall 6 and the outer periphery of the skirt 4.
However, this prior art arrangement has suffered from the drawback that, as the first recess 8 and the strip-like surface 10 are arranged at predetermined angles, the upper boundary 11 of the second recess 9 and the lower limit 12 of the first recess 8 define discrete stepped edges which inhibit the lubricating oil which is retained in the two recesses 8, 9 from being adequately supplied onto the thrust surfaces, and thus induce the situation wherein the oil film on the thrust surface breaks and increases the frictional sliding resistance. As a result, the piston is subject to increased heating. Further, as the recesses 8 and 9 reduce the amount of surface area of the thrust surface 7 which actually contacts the cylinder bore wall 6, the amount of force per unit area increases and in the event that an adequate amount of lubricant is not supplied, the tendency for overheating to occur increases.
Further, during reciprocation of the piston 1 in the cylinder bore, considerable side forces are applied, whereby the edges defined by the boundaries 11, 12 are brought into strong contact with the cylinder bore wall 6 and induce the generation of a relatively large amount of frictional resistance. This enhances the tendency for the piston to be subject to overheating.